This invention comprises a positioning mechanism for a centre wheel in an electronic timepiece with analog display and comprises a jumper spring of which one end is terminated in two inclined planes cooperating with the teeth of the centre wheel and the other end is friction mounted for rotation onto the base plate.
Such mechanism is known from the prior art and one may find application thereof in certain chronographs or certain electronic watches having analog display with the purpose of exactly positioning the seconds hand relative to the markings of the dial. A precise positioning of the hand relative to the index marking is still more desirable in electronic watches which may have only a minutes and an hours hands and wherein the minutes hand driven by a stepping motor steps through one division each minute. It will be understood that in such cases the eye of the user may well judge if the hand is properly positioned relative to the index marking which may represent a form of garantee as to the quality of manufacture of the timepiece.
The positioning of jumping seconds or minutes hands relative to a dial index depends on the play existing in the gear train found between the motor axis and the centre wheel which drives the hand, the trueness of the wheels and the regularity with which the index divisions are marked on the dial. In order to eliminate the first two of these criteria, one generally employs a positioning jumper spring which may act directly onto the teeth of the centre wheel which may drive either the minutes hand or the seconds hand. In such a manner the angular step of the hand will be always the same as that of the teeth of the wheel.
FIG. 1 shows the positioning mechanism for a centre wheel according to the prior art. This mechanism includes a jumper spring 1 formed by an elastic blade of which the first extremity may bear a ruby polyhedron 2 which cooperates with the teeth 3 of the centre wheel 4. In a simpler manner the end of the blade may simply be bent in order to form a triangle of which the two inclined planes cooperate with the teeth. The other extremity of the jumper spring 1 is fixed onto a collet 5 which may be friction fitted about an axis 6, the latter being fixed to a stud 7. In the stud are provided two oblong holes 8 and 9. Hole 8 may receive a circular pin 10 fixed to the base plate and the hole 9 a screw 11 screwed into the base plate. The centre wheel is driven by pinion 12 of an intermediate wheel 13 which meshes with the pinion 14 of a stepping motor. In order to regulate the positioning jumper spring it is necessary initially to turn the collet until the ruby 2 contacts the wheel 4 and to effect sliding of stud 7 in the axis of jumper spring 1 in a manner such that the inclined planes of the ruby bear between the outer extremities of two consecutive teeth of toothing 3, the gear backlash thus being divided in half on each side. As soon as the adjustment has been terminated the screw 11 may be tightened. Such operations are difficult and the means available are generally too coarse to assure an exact positioning. It is often necessary to repeat the operation several times since one movement may well influence the other.
It is the purpose of this invention to overcome these difficulties through proposing a positioning mechanism for the centre wheel having an easy precise adjustment and at the same time simplifying the design.
These purposes are attained through use of the claimed means.